Synthesizers are commonly used in radio communication devices to generate the carder signal having a desired frequency. In general and referring to FIG. 1, a communication device 100 includes a synthesizer 104 coupled to an oscillator 102. The synthesizer 104 produces a signal having a frequency determined by a controller 112. The output of the synthesizer 104 having a desired frequency is coupled to a buffer 106 and then applied to an amplifier 108 where it is amplified and transmitted via an antenna 110. In general, synthesizers operate in two distinct modes of operation. A first, adapt mode where the synthesizer has a wide loop bandwidth filter. This mode of operation utilizes a wide bandwidth to rapidly allow the synthesizer to settle on a desired frequency. Once the frequency has been reached, the synthesizer switches to a normal mode of operation which utilizes a narrow bandwidth filter to eliminate more of the undesired spurs. This second mode is the operating mode of the communication device 100.
Referring now to FIG. 2, a timing diagram of the operation of the synthesizer 104 in conjunction with the amplifier 108 is shown. The timing diagram shows the occurrence of several synchronous events. These events include synthesizer chip enable (CE), frequency pull, and transmit enable. The X axis represents time. With frequency information available on the data line 113 and the CE line active, the synthesizer 104 proceeds to produce a signal at its output. The frequency of this signal is pulled by an amount shown by waveform 204 in the adapt mode, which is the mode that the synthesizer initially attempts to produce the signal and lock to a desired frequency. It can be further seen by segment 206 that as the operation mode changes from adapt to normal, a small pull is again experienced. The transition of the amplifier 108 from the OFF state to the ON state occurs in the normal mode of operation, as shown by transient 212. When the transmit line goes high the amplifier 108 turns ON, generating a substantial pull 208 in the output frequency. This is a significant problem because this oscillation 208 will be transmitted as the amplifier 108 transmits all signals applied to its input. Normally, The frequency pull 208 will have an amplitude as high as 20 db from the base and a frequency pull of 200 Khz. Such frequency pull is a significant burden on communication devices having low frequency separation. It can be appreciated that such a substantial frequency drift causes disturbances in the adjacent channels, a remedy for which is highly desirable.
In the past, reverse isolation buffers have been used between the synthesizer and the transmitter chain to minimize these disturbances. These buffers generally demand high reverse isolation which is realized with high component counts. These buffers help in the alleviation of the frequency pull problem, however, they add cost to the overall cost of the product and require more volume of the available space in the communication device. A significant problem with these buffers is their layout requirements. Due to their sensitive operating environments such buffers must be carefully matched with stages to which they are coupled. Such matching requires careful layouts which may take several optimization iterations, further increasing product cost. With ever increasing demand in cost and size reductions, it is clear that a more substantial remedy to the frequency pull in a communication device is desired.